Ensure that your cupcakes are cool so that you can start decorating.
Start by making your buttercream: in a stand mixer, start beating butter, sugar and milk on medium speed, gradually increasing it to high. Add a couple of drops of violet gel food coloring and mix until completely tinted.
With an offset spatula, spread some buttercream on top of each cupcake.
Cut your marshmallows into small pieces. Place 2 or 3 on top of each iced cupcake.
Scoop the remaining buttercream into a piping bag and use Wilton tip #233 to frost each cupcake, being sure to cover it completely (the marshmallows are there to create some “hills and valleys”).
Place 2-4 eyes on each cupcake and enjoy your new, furry, purple monster!